In process measuring and automation technology, inline measuring devices, especially Coriolis mass flow measuring devices, are often used for measuring physical parameters of a medium flowing in a pipeline, parameters such as e.g. mass flow, density and/or viscosity. By means of a measurement pickup, or transducer, of vibration-type, through which the medium flows during operation, and by means of a measuring and operating circuit connected thereto, these devices effect reaction forces in the medium, forces such as e.g. Coriolis forces corresponding to mass flow rate, inertial forces corresponding to density, frictional forces corresponding to viscosity, etc., and produce, derived from these forces, measurement signals representing, respectively, the instantaneous mass flow rate, viscosity and/or density of the medium. Such inline measuring devices with a measurement pickup of vibration type, as well as their manner of operation, are known per se to those skilled in the art and are described comprehensively in e.g. WO-A 05/040734, WO-A 05/040733, WO-A 03/095950, WO-A 03/095949, WO-A 03/076880, WO-A 02/37063, WO-A 01/33174, WO-A 00/57141, WO-A 99/39164, WO-A 98/07009, WO-A 95/16897, WO-A 88/03261, US-A 2004/0200268, US-A 2003/0208325, U.S. Pat. No. 6,889,561, U.S. Pat. No. 6,840,109, U.S. Pat. No. 6,691,583, U.S. Pat. No. 6,651,513, U.S. Pat. No. 6,513,393, U.S. Pat. No. 6,505,519, U.S. Pat. No. 6,006,609, U.S. Pat. No. 5,869,770, U.S. Pat. No. 5,796,011, U.S. Pat. No. 5,616,868, U.S. Pat. No. 5,602,346, U.S. Pat. No. 5,602,345, U.S. Pat. No. 5,531,126, U.S. Pat. No. 5,301,557, U.S. Pat. No. 5,253,533, U.S. Pat. No. 5,218,873, U.S. Pat. No. 5,069,074, U.S. Pat. No. 4,876,898, U.S. Pat. No. 4,733,569, U.S. Pat. No. 4,680,974, U.S. Pat. No. 4,660,421, U.S. Pat. No. 4,524,610, U.S. Pat. No. 4,491,025, U.S. Pat. No. 4,187,721, EP-A 1 291 639, EP-A 1 281 938, EP-A 1 001 254 or EP-A 553 939.
For conveying the medium, the measurement pickups include, in each case, at least one measuring tube held in a, for example, tubular or box-shaped, support frame. The measuring tube, which has a straight tube segment, is caused, during operation, to vibrate, practically unifrequently, in a primary, wanted mode—driven by an electromechanical exciter mechanism—in order to produce the above-mentioned, reaction forces. For the registering of vibrations of the tube segment, especially inlet- and outlet-end vibrations thereof, the measurement pickups further include, in each case, a physical-to-electrical sensor arrangement reacting to movements of the tube segment.
In the case of Coriolis mass flow measuring devices, measurement of the mass flow rate of a medium flowing in a pipeline rests, for example, on the fact that the medium is allowed to flow through the measuring tube joined into the pipeline and oscillating, during operation, in the wanted mode laterally to a measuring tube axis, whereby Coriolis forces are induced in the medium. These, in turn, effect that inlet-side and outlet-side regions of the measuring tube oscillate with phases shifted with respect to one another. The size of these phase shifts serves as a measure of the mass flow rate. The oscillations of the measuring tube are, therefore, registered by means of two oscillation sensors of the aforementioned sensor arrangement. These sensors, which are spaced from one another along the measuring tube, convert the mechanical oscillations into oscillation measurement signals, from which the mass flow rate is derived from their phase shift with respect to one another.
U.S. Pat. No. 4,187,721 referenced above mentions further, that also the instantaneous density of the flowing medium is measurable by means of such inline measuring devices, and, indeed, on the basis of a frequency of at least one of the oscillation measurement signals delivered by the sensor arrangement. Moreover, most often also a temperature of the medium is directly measured in suitable manner, for example by means of a temperature sensor arranged on the measuring tube.
Additionally, straight measuring tubes can, as is known, when excited to torsional oscillations about a torsional oscillation axis essentially parallel to, or coinciding with, the longitudinal axis of the measuring tube, effect that radial, shearing forces are produced in the through-flowing medium, whereby, in turn, significant oscillatory energy is withdrawn from the torsional oscillations and dissipated in the medium. From this, a considerable damping of the torsional oscillations of the oscillating measuring tube results, so that, for maintaining the torsional oscillations, additional electrical exciting power must be fed to the measuring tube. Derived from an electrical exciting power required for maintaining torsional oscillations of the measuring tube, those skilled in the art can, in known manner, thus, determine, by means of the measurement pickup, also a viscosity of the medium, at least approximately; compare, in this regard, especially also U.S. Pat. No. 4,524,610, U.S. Pat. No. 5,253,533, U.S. Pat. No. 6,006,609 or U.S. Pat. No. 6,651,513.
A problem in the case of inline measuring devices of the described kind is to be seen, however, in the fact that the oscillatory characteristics of the measurement pickup and, to such extent, also the oscillation measurement signals derived from the oscillations of the measuring tube, are not only dependent on the primary, physical, measured variables of the medium, for example mass flow rate, density and/or viscosity, etc., and their changes during operation, but also, to a significant degree, on equally variable, secondary parameters, for example measuring-device-specific parameters or even parameters reflecting environmental and installation conditions. Representative examples of such changing, secondary parameters are the elastic and shear moduli of the materials used in the construction of the measurement pickup, as well as the geometry of the at least one measuring tube. The changes of the secondary parameters can, in such case, be both reversible, for example in the case of temperature-related, elastic deformations, and also, essentially, irreversible. Luckily, a large portion of such secondary parameters, or at least the influencing variables resulting in such changes, can be supplementally registered during measurement operation, and, to such extent, the influences of changes of such device and/or installation parameters on measurement accuracy can be largely compensated. This can, for instance, as proposed in U.S. Pat. No. 6,512,987, U.S. Pat. No. 4,768,384, EP-A 578 113, on the one hand, be implemented by using sensors additionally located in the inline measuring device, sensors such as e.g. temperature sensors, strain gages, acceleration sensors, pressure sensors, etc., and, on the other hand, be accomplished on the basis of the oscillation measurement signals themselves.
The principle of the compensation methods resting on the oscillation measurement signals is based essentially on the fact that, additionally to the primary, wanted modes causing the above-mentioned, reaction forces, other oscillation modes of most often, higher oscillation frequency are excited. These other oscillation modes serve mostly only as secondary, auxiliary modes. Thus, e.g. in WO-A 05/040734, U.S. Pat. No. 6,889,561, U.S. Pat. No. 6,557,422, U.S. Pat. No. 5,907,104, U.S. Pat. No. 5,831,178, U.S. Pat. No. 5,773,727, U.S. Pat. No. 5,728,952, and U.S. Pat. No. 4,680,974, in each case, an inline measuring device is disclosed for measuring at least one physical, measured variable of a medium conveyed in a pipeline. The inline measuring device comprises a measurement pickup of vibration-type and a measuring device electronics electrically coupled with the measurement pickup,                wherein the measurement pickup includes:        at least one measuring tube serving to convey the medium to be measured and communicating with the connected pipeline;        an exciter mechanism acting on the at least one measuring tube for causing the at least one measuring tube to vibrate,        which causes the measuring tube, during operation, to oscillate about an imaginary lateral oscillation axis, at least at times and/or at least in part, with first lateral oscillations having a first oscillation frequency; and        which causes the measuring tube, during operation, to oscillate about an imaginary lateral oscillation axis, at least at times and/or at least in part, with second lateral oscillations having a second oscillation frequency; as well as        a sensor arrangement for registering vibrations of the measuring tube and delivering oscillation measurement signals representing the oscillations of the measuring tube;        wherein the measuring device electronics delivers, at least at times, an exciter signal driving the exciter mechanism; and        wherein the measuring device electronics generates, by means of the oscillation measurement signals an/or by means of the exciter signal, at least at times, at least one measured value, which represents the at least one, physical, measured variable to be measured for the medium.        
On the basis of the oscillation measurement signals, the measuring device electronics determines, repetitively, the oscillation frequencies of the lateral oscillations of the measuring tube and determines and/or monitors, based thereon, at least one device- and/or installation-parameter of the inline measuring device, or detects at least one, unallowably high, measurement error.
As mentioned, among others, in WO-A 05/040734, also the formation of a deposit on the inside of the measuring tube wall, for example due to sedimentation, adhesion, or the like, can lead, to a considerable degree, to a degrading of the measurement accuracy of the inline measuring device, at least to the extent that this deposit formation is not taken into consideration in the determining of the measured value. Investigations have now shown, however, that an as early as possible detection of deposits on the measuring tube using multiple lateral oscillations can be associated with significant difficulties. This relates, in particular, also to the fact that, on the one hand, the density of the deposit naturally lies about in the range of the density of the medium, and, on the other hand, its influence on the lateral oscillations is approximately comparable with that of the medium being measured. As a result of this, a deposit, in the process of forming, can have essentially the same effect on the lateral oscillations as operational changes in the physical properties of the medium, especially changes in its density and/or viscosity.
Moreover, the case can also arise, that not only the at least one measuring tube of the inline measuring device becomes the subject of such a deposit, but, also, in particular, parts of the pipeline connected to the inline measuring device. This, in turn, can then, for example, lead to also other inline measuring devices and/or their inlet sections being affected by deposit formation, without that this would be recognizable, without more, by a corresponding self-validation on the part of the affected measuring devices.